staging: comedi: addi-data: Get rid of redundant ps_BoardInfo

The ps_BoardInfo pointer in the device private data is redundant as we
can just use the this_board macro to access the same data, as is done
elsewhere in the code.  Get rid of the pointer and change the code to
use the this_board macro instead.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>